The use of color centers in ionic crystals has been known for some time. A color center laser, for example, is a known light source that operates on a basis of random defects formed in an ionic crystal. See U.S. Pat. No. 5,889,804 entitled, “Artificial Color Center Light Source” which issued on Mar. 3, 1999 to Y. Takiguchi. In that patent there is described a color center light source where a color center is formed artificially. A predetermined single atom is removed from the surface of a defect-free ionic crystal so as to form a lattice defect. Optical transition of the defect is utilized so that it functions as a light source. In the past, these color centers have been formed by methods such as exposing the crystals to gamma radiation or heating in the presence of excess cations or other impurities. These methods cause anions to be displaced from the crystal lattice. The hole left by the cation can then be filled by an excess electron that is attracted to the void due to the positive ions surrounding it. The electron can then be treated as if in a potential well whose size is smaller than the wavelength of the electron; such a well has discrete energy levels which can be predicted quite easily. When an incident photon hits the trapped electron it will be absorbed if the energy of the photon is the same as the difference between the two energy levels of the electron in the well; this will also cause the electron to be excited into the higher energy state. In this way the electron can be used to absorb only select wavelengths of light that correspond to the energy levels in the well. Once the electron is in an excited state, the surrounding crystal will relax, thereby changing the energy gap between the excited and ground states of the potential well. When the electron decays back into the ground state it will emit a photon with different energy and therefore a different wavelength than the incident photon. This is commonly referred to as an F-center type color center, so called because the absorption of discrete wavelengths gives a unique color to the ionic crystal. In general, for most ionic crystals, an F-Center has an absorption peak within the visible light spectrum, however when an excited electron decays back to the ground state it does so over a smaller energy gap and emits light of a longer wavelength. There are other types of color centers such as FA, FB, F2+, and others which can be created through various types of annealing and bombardment by radiation. The other color centers are caused by various other impurities and dislocations present in the crystal and they will each absorb and emit at different areas of the spectrum.
Use of color centers has been employed in the prior art. See for example, the above-noted patent, U.S. Pat. No. 5,889,804. See also U.S. Pat. No. 4,990,322 entitled, “NACL:OH Color Center Laser” which issued on Feb. 5, 1991 to C. R. Pollock et al. and is assigned to Cornell. See also U.S. Pat. No. 4,839,009 entitled, “NACL:ON Color Center Laser” which issued on Jun. 13, 1989 to C. R. Pollock et al. See U.S. Pat. No. 4,638,485 entitled, “Solid State Vibrational Lasers Using F-Center/Molecular-Defect Pairs in Alkali Halides” which issued on Jan. 20, 1987 to W. Gellermann et al. See also U.S. Pat. No. 5,267,254 entitled, “Color Center Laser with Transverse Auxiliary Illumination” which issued on Nov. 30, 1993 to I. Schneider et al.
In most of these patents the color centers are created throughout the ionic crystal so that the whole crystal can be used to lase light. A notable exception to this is the point light source patent where a scanning electron microscope is used to create a single F-center dislocation to be used as a point light source. In all cases only a single type of ionic crystal is used so that there is only one absorption and emission peak.
It is an object of the present invention to provide an apparatus and a method of forming and utilizing defects formed in the ionic crystals.